1. Field of the Invention
The present invention relates to a fuel cell structure, and more particularly to a flat fuel cell assembly.
2. Description of Related Art
With the rapid development of industry, the consumption of conventional energy source such as coal, petroleum, and natural gas is increasingly high, and due to the limited storage of natural energy source, novel alternative energy source must be researched and developed to substitute the conventional energy source, and the fuel cell is taken as an important and practical choice.
In brief, the fuel cell is substantially a power generator that converts chemical energy into electric energy by utilizing the reverse reaction of the water electrolysis. The proton exchanging membrane fuel cell mainly includes a membrane electrode assembly (MEA) and two electrode plates. The MEA includes a proton conducting membrane, an anode catalyst layer, a cathode catalyst layer, an anode gas diffusion layer (GDL) and a cathode GDL. The anode catalyst layer and the cathode catalyst layer are respectively disposed on two sides of the proton conducting membrane, and the anode GDL and the cathode GDL are respectively disposed on the anode catalyst layer and the cathode catalyst layer. Furthermore, two electrode plates include an anode and a cathode, which are respectively disposed on the anode GDL and the cathode GDL.
Currently, the common proton exchanging membrane fuel cell is Direct Methanol Fuel Cell (DMFC), which directly takes the methanol aqueous solution as the source for supplying fuel, and generates currents through the relevant electrode reaction between methanol and oxygen, wherein the reaction formulas of the DMFC are shown as follows:anode: CH3OH+H2O→CO2+6H++6e−;cathode:3/2O2+6H++6e−→3H2O.
During the reaction, 1 mol of water is consumed at anode, three mol of water is generated at the cathode, and the water generated in the reaction must be removed immediately, and cannot be kept at the surface of the catalyst layer, in such a manner, the fuel cell can react continuously, thereby generating currents.
As for the water management in the fuel cell, many solutions have been proposed in this field. For example, early US Patent Publication No. 2005/0079398A1 (U.S. Pub. No. 2005/0079398A1) entitled “FUEL CELL” had disclosed that, additional devices such as pump, heat sink, and fan are used to remove the water generated in the fuel cell. However, this method increases the cost, and enlarges the volume of the whole assembly, which thus cannot be miniaturized. Furthermore, early US Patent Publication No. 2004/0209154A1 (U.S. Pub. No. 2004/0209154A1) entitled “PASSIVE WATER MANAGEMENT TECHNIQUES IN DIRECT METHANOL FUEL CELLS” had disclosed that, a hydrophobic material layer with micro-pores is disposed at the external side of the cathode, such that the water of cathode generates a back pressure there-between; then, the water is permeated to anode by utilizing the pressure difference between two sides of the proton conducting membrane, such that the water is recycled and used within the fuel cell. However, this method causes the problem that the micro pores are blocked by water or the water cannot be recycled. Thus, the above method is complicated in manufacturing, and the method even causes the problem that the air cannot be entered therein smoothly, thereby influencing the output power of the fuel cell.
Another water management in the fuel cell is provided in the Japan Patent Gazette WO 2006/101071, which had disclosed a fuel cell having an air chamber on the cathode side, and a humidity-holding sheet is filled in the air chamber. The humidity-holding sheet is mainly used to prevent the water generated at the cathode side from evaporating, and further increase the water storage in the cathode catalyst layer. Through the osmotic pressure phenomenon, the water generated in the cathode catalyst layer is promoted to move towards the anode catalyst layer.
Furthermore, the Japan Patent Gazette WO 2005/112172A1 had disclosed a fuel cell that vaporize the liquid fuel and then supply it to the catalyst layer, wherein the anode structure of the fuel cell must include a fuel tank, a fuel vaporizing layer and a vaporized fuel collector and the like, so as to vaporize the liquid fuel, and the cathode includes a humidity-holding layer with an appropriate moisture permeability and air permeability. However, the humidity-holding layer is made of a porous material with uniform micro pores, and during practical use, the micro pores are easily to be blocked due to the condensation of moisture, such that the gas cannot enter therein, thereby influencing the output power of the fuel cell.
In view of the above, the water management is a crucial technique for the fuel cell, which has become a key issue being researched and developed in this field.